1. Field of the Invention
The present invention relates to a method of forming a pattern of an inorganic material film, and specifically, to a method of forming a pattern of an inorganic material film having functionality such as piezoelectric ceramics to be used in micro devices so called micro electrical mechanical systems (MEMS). Further, the present invention relates to a structure containing a pattern of an inorganic material film formed by using such a method.
2. Description of a Related Art
In recent years, research and development of micro electro mechanical systems (MEMS) applying semiconductor manufacturing processes have been increasingly made. Among the systems, the piezoelectric MEMS using a piezoelectric film as a functional film attracts attention as a high-power actuator, and is employed for micropumps, micro-cantilevers, micro ultrasonic transducers, and so on. Here, the functional film refers to a main part (typically, a layer sandwiched between electrodes) that exerts a function of an element like a dielectric film in a multilayered capacitor and a piezoelectric film in a piezoelectric actuator.
In the MEMS, it is important to finely pattern-forming the functional film. However, conventionally, for functional films having a thickness of about 5 μm or more, there are not many appropriate fine patterning methods and appropriate materials to be employed therein. For example, the etching method or the ion beam method has disadvantages that the substrate and surrounding elements are damaged, the process time is long, and the manufacturing cost is high, and therefore, the application to pattern-forming of the functional film is not so practical. Further, although the liftoff method is higher in general versatility because the material dependence is lower than that of the etching method, the photoresist generally used as a sacrifice layer in the liftoff method deforms or burns dry at about 150° C., and the method is not applicable to the case where the process temperature (deposition temperature) becomes higher then 150° C.
In the liftoff method, not only the photoresist but also silicon oxide (SiO2), polysilicon, aluminum (Al), and so on are used as the sacrifice layer, and in such a case, the constraint on the deposition temperature is eased. However, the hydrofluoric acid used when removing silicon oxide and the acid or alkali solution used when removing aluminum may cause damage to the functional film. Further, since the xenon fluoride (XeF2) gas used when removing polysilicon is expensive, the manufacturing cost rises. Furthermore, when the functional film is thicker than the sacrifice layer, the etchant penetration is blocked by the functional film and hard to reach the sacrifice layer, and thus, the removal of the sacrifice layer is difficult.
As a related technology, Japanese Patent Application Publication JP-P2001-347499A discloses a method of manufacturing a microdevice including the steps of forming a die by recessing a groove or pore pattern deeper than a desired functional material layer on a silicon layer, depositing a functional material in the grooves or pores of the pattern of the die in a thinner thickness than that of the silicon layer, obtaining a pattern of the functional material layer by removing the die. That is, in JP-P2001-347499A, the thickness of the sacrifice layer (Si layer) is slightly thicker than the functional film (PZT layer), and thus, the selective etching of the sacrifice layer with the etching gas is promoted through the space formed by the level difference between the films (paragraph 0027).
However, when the sacrifice layer is made thicker, the functional film may be affected by the stress of the sacrifice layer. Further, the longer time is required for the deposition and removal processes of the sacrifice layer, and as a result, the manufacturing cost increases. Furthermore, JP-P2001-347499A is not so practical because the photoresist and the organic compound film with poor heat resistance and silicon requiring an expensive etching gas such as xenon fluoride are used for the sacrifice layer.
On the other hand, Japanese Patent Application Publication JP-P2004-282514A discloses formation of a piezoelectric thin film resonator having an air-gap acoustic insulation structure on a semiconductor integrated circuit by using germanium (Ge) as a material of a sacrifice layer and etching the sacrifice layer to remove it by using a hydrogen peroxide (H2O2) solution, in order to form the air-gap acoustic insulation structure without property degradation due to damage on a CMOS circuit. However, when germanium is used for the sacrifice layer, some design ideas of forming a level difference on the substrate or the like is required for ensuring that the etchant reaches the sacrifice layer.